This invention relates to constant current regulators, and more specifically to a high-power regulator controlled by a programmable logic for airport lighting applications.
Approach lights for airport runways typically include sets of high-wattage lamps connected in series in a lighting loop. In order to maintain a uniform intensity throughout the loop regardless of supply voltage variations, and to allow selected changes of intensity to cope with various weather and natural light conditions, the lighting loop has to be supplied with an adjustable constant current that is unaffected by supply voltage variations or other electrical disturbances.
In addition, airport lighting is subject to strict FAA regulations which require, for example, minimization of switching harmonics and minimization of inductive loading of the power supply. Switching harmonics are undesirable both as a reflection into the power supply, and in the lighting loop. In the latter, the skin effect from high power harmonics can require the use of heavier copper cables (which can be quite long in airport applications), and the radiation of harmonics from the lighting loop can interfere with sensitive communication systems such as instrument landing systems.
The individual lamps of the loop are typically fed from the loop through isolation transformers. If a lamp burns out, the isolation transformer primary winding acts as an inductor and puts a substantial inductive load on the circuit. For that reason, a shorting device is mounted across the secondary of the isolation transformer. When the lamp fails (i.e., opens up), the shorting device is activated to keep the integrity of the loop intact.
With conventional analog control circuitry, control of an airport lighting constant current regulator is feasible but is not very flexible or efficient. It is therefore desirable to give the regulator a maximum of flexibility, e.g., automatic power reduction on power-up, power-down and in error conditions.
Another problem of the prior art is that regulators exceeding about 30 kW power capacity usually required oil cooling, which was expensive and environmentally undesirable. Also, a regulator adaptable to a wide range of power outputs generally required switchable taps on the main transformer windings that required resetting the transformer when changing the load.
The present invention solves the problems of the prior art by providing a constant current regulator using an air-cooled ferroresonant transformer that maintains a good power factor and efficiency over the entire 30 kW to 50 kW output power range without requiring any tap switching, in conjunction with a control system using a programmable logic to track circuit conditions and user interfaces, and to take appropriate control actions in response thereto.
Each and every feature described herein, and each and every combination of two or more of such features, is included within the scope of the present invention provided that the features included in such a combination are not mutually inconsistent.
These and other aspects of the present invention are set forth in the following detailed description and accompanying claims, particularly when considered in conjunction with the accompanying drawings in which like parts bear like reference numerals.